Cassini's camera had already imaged an impressive cloud hovering over Titan's south pole at an altitude of about 300 kilometres.

However, that cloud, first seen in 2012, turned out to be just the tip of the iceberg. A much more massive ice cloudsystem has now been found lower in the stratosphere, peaking at an altitude of about 200 kilometres, NASA said.

The new cloud was detected by Cassini's infrared instrument – the Composite Infrared Spectrometer, or CIRS – which obtains profiles of the atmosphere at invisible thermal wavelengths. The cloud has a low density, similar to Earth's fog but likely flat on top.

For the past few years, Cassini has been catching glimpses of the transition from fall to winter at Titan's south pole – the first time any spacecraft has seen the onset of a Titan winter.

Because each Titan season lasts about 7-1/2 years on Earth's calendar, the south pole will still be enveloped in winter when the Cassini mission ends in 2017.

"When we looked at the infrared data, this ice cloud stood out like nothing we've ever seen before," said Carrie Anderson of NASA's Goddard Space Flight Center in Maryland.